Electroacoustic transducers, and in particular electrodynamic transducers, are widely used in telecommunications equipment such as wired and mobile telephones, where small size is a requirement. Traditional electrodynamic microphones and loudspeaker transducers used in e.g. mobile telephones are rotational symmetric and have a circular disc or ring-shaped permanent magnet, which is magnetised in the axial direction of the magnet. A magnetic circuit of magnetically soft iron or other suitable material define a ring-shaped gap with a radially oriented magnetic field generated by the magnet. A diaphragm carries a ring-shaped coil of electrically conducting wire situated in the gap.
If the inner and outer members defining the gap are not perfectly coaxial, the gap will not have a uniform width resulting in a distorted distribution of the magnetic field along the gap. A coil carrying electric currents at audio frequencies in such a distorted magnetic field will tend not to move in a linear movement but rather to tilt, which causes linear and nonlinear distortion.
In such transducers the magnetic field in the ring-shaped gap is radially oriented, whereby the magnetic field is inherently stronger at its inner limit than at its outer limit. A not perfectly centred coil will cause the same distortion as mentioned above.
Another disadvantage of traditional transducers with ring-shaped magnetic gap is related to the fact that they require a considerable building height since the magnetic circuit requires a bottom piece. For applications in miniature equipment, such as hearing aids or mobile phones, the dimensions of each component is a crucial parameter whereby the use of traditional transducers with ring-shaped gaps within e.g. hearing aids becomes a very difficult task.
It is an object of the present invention to solve the above-mentioned problems/disadvantage of traditional transducers that related to the fact that they are not suitable for miniature applications—such as being used in hearing aids or mobile phones.
It is another object of the present invention to solve the above-mentioned disadvantage of traditional transducers that relates to the fact that small imperfections in the magnetic field causes distortion of the acoustic output (or electrical output in case of the transducers used as microphones).